Toxicodendron diversilobum (syn.
Rhus diversiloba; Western Poison-oak or
Pacific Poison-oak) is a plant best known for its
ability to cause allergic rashes after contact. Western Poison-oak
is found only on the Pacific Coast of the United States and
of Canada. It is extremely
common in that region, where it is the predominant species of the
genus; the closely related Atlantic
Poison-oak (T. pubescens) occurs on the Atlantic Coast. The
hyphenated form "Poison-oak" is used, rather than "Poison Oak" to
clearly indicate that it is not a variety of oak, just as "Poison-ivy" is not a
variety of ivy.

Contents

Appearance

Western Poison-oak is extremely variable in growth habit and
leaf appearance. It grows as a dense shrub in open sunlight, or as a climbing vine in shaded areas. Like Poison ivy, it reproduces
by creeping rootstocks or by seeds.[1] The
leaves are divided into three leaflets, 3.5 to 10 centimeters long, with
scalloped, toothed, or lobed edges- generally resembling the leaves
of a true oak, though the Western Poison-oak leaves will tend to be
more glossy. Leaves are typically bronze when first unfolding,
bright green in the spring, yellow-green to reddish in the summer,
and bright red or pink in the fall. White flowers form in the
spring and, if fertilized, develop into greenish- white or tan
berries. Toxicodendron diversilobum is winter deciduous, so that after
cold weather sets in the stems are leafless and bear only the
occasional cluster of berries. Without leaves, poison oak stems may
sometimes be identified by occasional black marks where its milky
sap may have oozed and dried.

Botanist John Howell observed that Toxicodendron
diversilobum's toxicity obscures its merits. "In spring the
ivory flowers bloom on the sunny hill or in sheltered glade, in
summer its fine green leaves contrast refreshingly with dried and
tawny grassland, in autumn its colors flame more brilliantly than
in any other native, but one great fault, its poisonous juice,
nullifies its every other virtue and renders this beautiful shrub
the most disparaged of all within our region."[2]

Green phase

Red phase

Berries

Pacific Poison-oak

Ground variation

Climbing variation

Habitat

Western Poison-oak occurs only on the Pacific Coast, where it is
common, and ranges from southern Canada to the Baja California peninsula. It
is one of California's most prevalent woody shrubs but also climbs,
vine-like, up the sides of trees,
and can be found growing as single stems in grassland--often as
part of early stage succession where woodland has been removed, and
serving as a nurse plant for other species. The plant often occurs
in California oak woodlands and Douglas-fir
(Pseudotsuga) forests. Along the Avenue of the Giants in
northern California, the vine form may be seen climbing many feet
up the trunks of Coast Redwoods (Sequoia sempervirens). It can also be
found in damp, shady areas near running water and out of direct
sunlight. Any trail leading to a waterfall on California's coast
may likely be home to western poison-oak. Western poison-oak can
also be found in some inland mountain ranges such as the
Cascades.

Toxin

Western Poison-oak leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. Around
15%[3] to
30%[4] of
people have no allergic response, but most if not all will become
sensitized over time with repeated or more concentrated exposure to
urushiol.

Effects

Effects of Poison Oak are similar to those of Poison Ivy. It
first causes severe itching, evolves into inflammation, non-colored
bumps, and then blistering when scratched.